In recent years, a lithium secondary battery that employs a non-aqueous electrolyte and is charged and discharged through transfer of lithium ions between the positive electrode and the negative electrode has been used as one of new secondary batteries having a high power and a high energy density.
Such a lithium secondary battery that employs a material that alloys with lithium as a negative electrode active material is known. However, such an active material that alloys with lithium undergoes volume expansion and contraction during occlusion and release of lithium. Accordingly, the active material turns into fine powder or the active material becomes separated from a current collector during charging and discharging. Thus, the current-collection capability is degraded and the charge-discharge cycle characteristic becomes poor, which is problematic.
Patent Literature 1 states that, in a lithium secondary battery employing silicon and/or a silicon alloy as an active material, a mixture layer containing the active material and a binder is disposed through sintering in a non-oxidizing atmosphere so that the active material mixture layer is tightly bonded to the current collector and a good charge-discharge cycle characteristic is achieved.
However, when such a lithium secondary battery is configured as a wound-type battery in which a laminate body having a stack of a negative electrode, a separator, and a positive electrode is spirally wound, the whole electrode expands and contracts during charging and discharging and the electrode cannot withstand the expansion and breaks, which is problematic.
In the present invention, as described below, a tapering portion is formed in an outer end portion of the negative electrode. Existing techniques of forming a tapering portion in an end portion of an electrode are described in the following Patent Literatures 2 to 4.
Patent Literature 2 discloses that the inner end portion of a positive or negative electrode is formed as a tapering shape so that separation of a porous layer composed of a filler and a binder and disposed on the positive or negative electrode is suppressed.
In Patent Literature 3, the inner end portions of electrodes are formed as tapering shapes so that the central portion of the electrode body is formed as a smooth circular shape to thereby suppress damage to the separator.
In Patent Literature 4, the whole circumference of a positive electrode plate is subjected to chamfering so that the positive electrode plate is pressed by the separator and gaps are not generated.